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FXUS61 KPBZ 250256 AFDPBZ

Area Forecast Discussion National Weather Service Pittsburgh PA 1056 PM EDT Wed Apr 24 2024

SYNOPSIS
Colder air filters in tonight with decreasing cloud cover.
Temperatures rebound tomorrow with marginal wire weather. The weekend remains warm with intermittent precipitation chances.

NEAR TERM /UNTIL 6 AM THURSDAY MORNING/
KEY MESSAGES:

- Freeze warning and frost advisories are in effect north and west of the Pittsburgh metro beginning at midnight.

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Update...
Adjusted overnight lows and sky cover based on the latest satellite, surface observations, and near term model output. It appears the low level moisture will remain locked below a subsidence inversion for a good portion of the area overnight.
Clearing will likely take place under dry advection, mainly north and west of Pittsburgh, where frost and freeze conditions are likely.

Previous discussion...
The forecast remains on track late this evening with ongoing cold advection in northwest flow reinforcing saturation and cloud coverage in the low-levels through the early overnight hours before drier air advects in from the north and low level flow veers more northwesterly cutting off the lake source.

Right now, there is high confidence in a freeze (80% to 100%), potentially even hard freeze (60% to 80%), north of I-80 where clearing is expected early tonight and radiational cooling will develop. Elsewhere, the degree of cooling will depend on clearing which remains the forecast challenge for tonight.

The freeze warning was hoisted for where probabilities of subfreezing temperature are greater than 50%. For the areas in the warning south of I-80, clearing is not guaranteed with large spread between the low and high end of cloud cover in the latest hi res ensemble guidance. Thus, there is a failure mode for the warning. On the contrary, clearing may go as far as covering areas in the frost advisory, where freezing temperatures are also not out of the realm of possibility. Thus, there is generally high uncertainty in temperatures in-between I-80 and the advisory area; a sharp temperature gradient may orient along the cloud extent maxima. Elsewhere, clearing is not favored with probability for less than 50% cloud cover generally sitting below 50%, thus frost formation and significant cooling are also not favored.

SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY NIGHT/
KEY MESSAGES:

- Dry conditions with a warming trend are expected through Friday; showers return Friday night.

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Thursday will bring mundane weather. In the morning and early afternoon, mixing will act to gradually erode the lingering stratocumulus deck. This will lend to a rebound in temperature under clear skies and high pressure.

The NBM clear sky under-mixing bias was accounted for in the forecast as daytime highs were adjusted upward a couple of degrees and daytime dews minimize at the NBM 10th percentile.
This has dropped relative humidities into the upper 20s and lower 30s. Dead and fine fuel moistures may dip below 10% during the day, although winds will remain calm to light. This will lead to another day of marginal fire weather, with conditions most favorable in northwestern Pennsylvania.

A temperature and dewpoint push is expected through Friday as the high moves east and a southerly component to the flow reestablishes. Low-QPF rain showers are possible Friday night and early Saturday morning as a warm front approaches from the south.

LONG TERM /SATURDAY THROUGH WEDNESDAY/
KEY MESSAGES:

- Much warmer through the weekend - Highest chance for showers on Saturday, and again Monday through Tuesday -------------------------------------------------------------------

Model ensembles indicate the initial shortwave and warm front supported rain will continue to cross the region on Saturday. The front is expected to lift north of the region Saturday night, as a strong ridge builds across the SE CONUS to the Upper Ohio Valley region.

Mainly dry weather is expected Saturday night and Sunday with the ridge axis across the area. There could be enough diurnally driven instability to result in an isolated afternoon thunderstorm Sunday, though most of the day should be dry, with temperatures in the upper 70s to the mid 80s. 1000-500mb height approach 583 dam Sunday into Monday, with 850 mb temperatures peaking around 14 deg C.

Model ensembles indicate the ridge should slowly shift eastward to the East Coast late Monday and Monday night, as the next shortwave trough and surface cold front approach from the Midwest. Scattered showers and thunderstorms are possible Monday with building diurnal instability in moist SW flow ahead of the trough. More numerous showers and possible thunderstorms are expected Tuesday as the trough/cold front cross the region.

AVIATION /03Z THURSDAY THROUGH MONDAY/
Northwest flow and ongoing shortwave troughing are keeping MVFR stratocumulus (IFR at FKL) in place across much of the region, with locations near/south of the Mason-Dixon Line escaping restrictive impact for now. As a vorticity maximum sinks south and east this evening, lower clouds will accompany it, providing MVFR ceilings to MGW by late evening. Northwest wind will continue to diminish with time, eventually going light by midnight.

MVFR ceilings will largely persist through the night and past sunrise with moisture trapped under an inversion. FKL will continue to deal with IFR and potentially LIFR ceilings overnight as the low levels cool. Past sunrise, as high pressure tracks to the north, the low cloud deck will mix out, leading to VFR conditions by midday that will linger through the afternoon, along with light wind.

Outlook
VFR conditions are expected Thursday and Friday.

The probability of showers and isolated thunderstorms increases Saturday. This will come with additional restrictions likely.

PBZ WATCHES/WARNINGS/ADVISORIES
PA...Freeze Warning until 10 AM EDT Thursday for PAZ007>009-013>016- 022-077.
Frost Advisory until 10 AM EDT Thursday for PAZ020-078.
OH...Freeze Warning until 10 AM EDT Thursday for OHZ039>041-048.
Frost Advisory until 10 AM EDT Thursday for OHZ049-050-057-058.
WV...Frost Advisory until 10 AM EDT Thursday for WVZ001.

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